This application is a national stage application of PCT/FR99/03041, filed Dec. 7, 1999.
The subject of the present invention is a process and an apparatus for the continuous coating of at least one metal strip with a thin fluid film of crosslinkable polymer containing neither non-reactive solvent nor diluent.
Thermally crosslinkable polymers such as, for example, thermosetting polymers, or physically crosslinkable polymers such as, for example, photocurable polymers, are known.
There is a wide variety of thermosetting organic coatings which are continuously applied to metal substrates.
In most cases, these are complex formulations which combine, in a solvent or aqueous medium, a system of prepolymer functional organic binders, a crosslinking system and additives, such as pigments or fillers, various formulation adjuvants.
Various processes are also known for applying a thermoplastic or thermosetting organic coating to a bare or coated metal strip.
The application of organic coatings such as, for example, liquid paints or varnishes is usually carried out by roller coating these liquid coatings in the state of a solution or of a dispersion in an aqueous or solvent medium.
To do this, the liquid coating is deposited on a metal strip by predosing the solution or dispersion using a system comprising two or three rollers and by transferring some or all of this liquid coating thus predosed onto an applicator roller in contact with the surface of the metal strip to be coated.
The transfer is performed either by friction of the applicator roller on the metal strip, the two surfaces in contact running in opposite directions, or by contact in the same direction.
An advantageous trend in the technology of continuous application of crosslinkable polymer coatings, such as thermosetting paints or varnishes for example, to a metal strip consists in depositing this coating without the use of a solvent or a diluent.
Several alternatives have been proposed for producing and applying organic coatings without the use of a non-reactive solvent or diluent.
However, the processing poses two problems, namely that of producing a homogeneous dispersion of the fillers and pigments in the binder system and that of applying the product thus obtained.
One of the techniques used for applying such coatings consists in applying the organic coating in the form of a powder.
Another technique for applying a liquid coating to a metal strip is known, this technique using a heating tank, usually called a melting kettle, provided in its lower part with an orifice from which the liquid polymer contained in the tank flows.
Placed below this tank are two parallel rolls in contact with each other and the metal strip to be coated moves along beneath these rolls.
The liquid polymer is poured into the nip of the rolls, then flows between the said rolls and is deposited on the metal strip.
However, this technique has drawbacks stemming from the fact that the polymer can be only slightly reactive on account of its relatively long storage time in the heating tank and from the fact that it does not allow the thickness of the coating film on the metal strip to be controlled and consequently does not allow a thin homogeneous coating to be obtained.
To produce thin coatings of viscous organic products, another technique consists in using the extrusion of the organic coating in the fluid state and in applying this coating to a substrate by extrusion coating or by lamination.
It is common practice to apply a thin organic coating, particularly of thermoplastic polymers, by extrusion coating to flexible substrates, such as paper, plastic films, textiles or even thin metal substrates, such as packaging materials.
The molten coating is applied by means of a rigid sheet die or of a nozzle positioned in direct contact with the substrate.
The pressure exerted by the die on the substrate derives from the viscosity of the melt. Thus, any possibility of correcting the discrepancies in flatness of the substrate, by pressing the latter against the back-up roll, is very limited.
This extrusion lamination technique requires there to be strict parallelism between the edges of the die and the substrate, and this substrate must either be perfectly plane or deformable, in order to allow the formation of a thin coating of uniform thickness.
This is because the thickness of material deposited is controlled by the gap and the pressure between the die and the substrate, thereby imposing strict parallelism between these two elements when it is desired to apply very small thicknesses.
This condition cannot be met in the case of steel strip having a thickness of between 0.3 and 2 mm, which is too rigid and has an insufficient flatness or thickness uniformity to allow such precise adjustment of the gap between the die and the substrate, particularly in the case of wide strip.
The technique of extrusibn-laminating a uniform layer of fluid coating on a substrate uses the drawing, beneath the die, of a fluid sheet at the exit of a sheet die, this sheet then being pressed against the substrate with the aid, for example, of a cold roller or of a rotating bar, or else by an air knife or an electrostatic field.
In this case, the thickness of the fluid sheet is controlled by the flow rate of the material in the die section and by the speed of the substrate.
In the event of the fluid sheet sticking to the press roll, the sheet would then separate into two parts within its thickness, one part being applied to the substrate and the other part remaining applied to the roll. This separation of the sheet therefore means that the transfer is not complete and the coating obtained on the substrate does not have a satisfactory surface appearance nor a uniform thickness.
In order to prevent the fluid sheet from sticking on the pressing roller, the latter must have a perfectly smooth and cooled surface.
The pressing pressure must however be low enough to prevent the formation of a calendering bead and consequently, this mode of transfer does not make it possible to compensate for any thickness variations and discrepancies in flatness in the case of a rigid substrate.
This technique of applying the coating with the formation of a free strand at the exit of the extrusion die makes it possible to avoid the problems of coupling between the die and the rigid substrate, but it causes application instabilities if the length of the free strand fluctuates and it is difficult to carry out with thermosetting systems having a viscosity of less than 2000 Pa.s because of the difficulties in achieving uniform drawing and good pressing.
In general, in the various known techniques mentioned above, the continuous application of a thin organic coating to metal substrates is carried out with low contact pressures, insufficient to allow production of a thin uniform coating applied homogeneously to rigid substrates which may have flatness and thickness-heterogeneity discrepancies.
These various application techniques do not make it possible to compensate for the variations in thickness of the metal substrate, which variations consequently cause unacceptable fluctuations in the thickness of the coating, especially if the substrate is formed by a metal strip which exhibits significant surface roughness and/or corrugations of amplitude equal to or greater than the thickness of the coating to be produced on the said metal strip.
Moreover, these various application techniques do not make it possible to allow for variations in the width of the substrate nor variations in the transverse positioning of this substrate, so that the coating cannot be deposited uniformly over the entire width of the substrate.
Finally, during application of the coating, air microbubbles may be trapped between the coating and the substrate, which is to the detriment of homogeneous application and to the surface appearance of this coating.
Thus, the continuous application of a thin uniform coating of crosslinkable polymer to a metal strip therefore causes problems because this metal strip has flatness and thickness discrepancies as well as significant roughness and/or corrugations of amplitude equal to or greater than the thickness of the coating film to be deposited on the said strip, even when this strip is pressed with a high force against a uniform roll.
In addition, the various techniques used hitherto do not make it possible to apply, to a metal strip, a thin coating of crosslinkable polymer containing neither non-reactive solvent nor diluent, meeting two contradictory requirements, namely hardness and deformability.
This is because, after crosslinking, the polymer coating must be sufficiently hard while still being deformable in order to allow the forming of metal sheet thus coated without causing degradation or debonding of the coating.
Now, it is known that increasing the molecular mass of the crosslinkable precursors of the polymer is highly favourable to obtaining a final coating which is both hard and deformable.
However, increasing the molecular mass of the precursors has a very unfavourable effect on the viscosity of a polymer containing neither non-reactive solvent nor diluent, this not being favourable to transfer and application of the sheet onto the metal
The object of the invention is to avoid these drawbacks by providing a process and an apparatus for the continuous coating of at least one metal strip with a thin fluid film of crosslinkable polymer containing neither non-reactive solvent nor diluent, and the softening temperature of which is higher than 50xc2x0 C., making it possible to obtain a coating of uniform thickness of a few microns to a few tens of microns applied homogeneously to this strip, while preventing air microbubbles from being trapped between the film and the metal strip and obviating discrepancies in the flatness and roughness of this strip as well as allowing application on part or all of the coating, despite the fluctuations in width and transverse positioning of this strip. The subject of the invention is therefore a process for the continuous coating of at least one metal strip with a fluid film of crosslinkable polymer containing neither non-reactive solvent nor diluent and the softening temperature of which is higher than 50xc2x0 C, the said film having a thickness of less than that of the metal strip, characterized in that:
the metal strip is made to run continuously over at least one back-up roll;
a sheet of the said crosslinkable polymer is formed, by forced flow at a temperature above the softening temperature of the crosslinkable polymer, on an applicator roll having a deformable surface, the said crosslinkable polymer having a melt viscosity greater than 10 Pa.s under the conditions of formation of the said sheet, the temperature of formation of this sheet being below the crosslinking onset temperature of the crosslinkable polymer and the said applicator roll being driven in rotation in the opposite direction to the direction in which the metal strip runs;
the crosslinkable polymer film is formed on the applicator roll;
the film is transferred, thicknesswise, from the applicator roll onto the metal strip, by compressing this metal strip between the back-up roll and the applicator roll, in order to obtain a coating of homogeneous thickness.
The subject of the invention is also an apparatus for the continuous coating of at least one metal strip with a fluid film of crosslinkable polymer containing neither non-reactive solvent nor diluent and the softening temperature of which is higher than 50xc2x0 C, the said film having a thickness of less than that of the metal strip, characterized in that it comprises:
means for continuously driving the metal strip;
at least one back-up roll supporting the metal strip;
means for forming, on an applicator roll having a deformable surface, by forced flow at a temperature greater than the softening temperature of the crosslinkable polymer, a sheet of the said crosslinkable polymer having a melt viscosity greater than 10 Pa.s under the conditions of formation of the said sheet, the formation temperature of this sheet being below the crossslinking onset temperature of the crosslinkable polymer and the said applicator roll being driven in rotation in the opposite direction to the direction in which the metal strip runs and forming the crosslinkable polymer film;
and means for compressing the metal strip between the back-up roll and the applicator roll in order to completely transfer, thicknesswise, the said film from the applicator roll onto the metal strip and to obtain a coating of homogeneous thickness.